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"He was someone who left for work today expecting to return home at the end of his shift, and he had every right to expect that would happen," Britain's most senior counter-terrorism police officer Mark Rowley said.

Another victim has been identified as Aysha Frade, a 43-year old teacher. The mayor of Betanzos in Spain, Ramon Garcia Vasquez, confirmed her death. She had lived in London for several years with her Portuguese husband, he said, but she had relatives in Betanzos.

"We are totally overwhelmed by the news and we send our condolences to the people in London," Garcia Vasquez told CNN.

Little is known about the third person killed, other than he was a man in his mid-50s.

'A lovely man'

After the police officer Palmer was stabbed, bystanders rushed to his aid. As he lay bleeding on the cobblestone street, Conservative MP Tobias Ellwood gave him mouth-to-mouth resuscitation, to no avail.

"Keith Palmer was killed while bravely doing his duty, protecting our city and the heart of our democracy from those who want to destroy our way of life," London Mayor Sadiq Khan said.

"He personifies the brave men and women of our police and emergency services who work around the clock to keep us safe -- tonight all Londoners are grateful to them."

At Scotland Yard, headquarters of London's police force, flags were flying at half-staff on Wednesday evening, in honor of Palmer and the other victims.

His death drew tributes from politicians including Conservative James Cleverly, who said he served with Palmer in the Royal Artillery before he became a "copper."

"A lovely man, a friend. I'm heartbroken," Cleverly said on Twitter.

Tourists struck

Around 40 other people were injured, police have said, among them are five South Korean sightseers, an Australian resident, three French students and a Chinese tourist.

A man and three women in their 50s and 60s from South Korea suffered fractures, while another woman in her 60s needed surgery after sustaining a serious head injury.

In Australia, Attorney General George Brandis told the Senate a female permanent resident from South Australia had been hospitalized after the attack.

The three French students were from Saint-Joseph school in Concarneau in western France, according to the French Foreign Ministry. They were on a school trip to London. There was no word on the extent of their injuries.